In the past, all-terrain vehicles (hereinafter ATVs) such as four wheeled buggy vehicles, buses, trucks, a variety of construction machines, and various industrial machines have employed an infinitely variable transmission which is called a hydro mechanical transmission (hereinafter HMT). As described in U.S. Pat. No. 4,341,131 and Japanese Patent Kokai No. S54-35560, the HMT is implemented by combining a hydro static transmission (hereinafter HST) that makes utilization of operating-oil hydrostatic energy and a mechanical transmission (hereinafter MT) through a differential gear mechanism such as a planetary gear mechanism. The HMT is so configured as to perform non-stage, continuous power transmission by synthesizing input on the HST side and input on the MT side in the differential gear mechanism.
A technique has been known in the art in which an HMT is mounted integrally on the engine. One such configured power unit is described for example in Japanese Patent Kokai No. H07-113454 (U.S. Pat. No. 3,016,057). As a way of realizing integration of an engine and an HMT, there are some techniques. In one technique, an HMT casing is mounted fixedly to an engine crank case. In another technique, an engine and an HMT are arranged within a common casing.
For the case of integrating together an engine and an HMT in the way as described above, it is possible to share a common oil as an engine lubricating oil and as an HST operating oil. Further, such a common oil is possibly used even in such cases that only the HST is used as a transmission device and is integrated with the engine.
In these cases, however, it is difficult to have control of the temperature of the common oil. In other words, generally the oil temperature is higher on the engine side than on the HST side. Therefore, at the time when the common oil, which has risen in temperature, is circulated to the HST, its viscosity is too low to serve as an operating oil for the HST. This causes considerable leakage of the common oil, thereby producing the possibility that there is a drop in the mechanical efficiency of power transmission. Further, there is the possibility that there occurs a seizing due to overheat.
Bearing in mind the aforesaid problems, the invention was made. Accordingly, an object of the invention is to prevent a drop in the efficiency and the occurrence of seizing on the HST side in such cases that the engine and the HST share a common oil.